Efficient High Power Flash Light

AND8135/D
Efficient High Power
Flash Light
Prepared by: Michael Bairanzade
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
INTRODUCTION
Depending upon the type of flash involved, the amount of
energy stored into capacitor C1 can be a low 10 Joule (small
camera) to thousand of Joule for professional applications.
Although the xenon lamps dominate the standard camera
market, they are not applicable in the portable phone. As a
matter of fact, the large reservoir capacitor and high voltage
associated with the xenon flash make such a concept not
suitable for the cellular phone equipments. The
semiconductor based White LED devices provide the right
choice when limited flash light becomes necessary to
illuminate a photographic scene. This paper depicts the
basics of the xenon concept and details a typical White LED
flash application.
+260 V
XENON TUBE
3
TRIGGER
C2
100 nF/
300 V
A low pressure of a rare gas mixture fills a glass envelope
with two ends electrodes on both sides. In steady state, the
voltage across the electrodes is set to a value well below the
trigger voltage as depicted Figure 1. At this point, no current
flows and the system is stable until a trigger voltage is
applied to the third electrode. This high voltage pulse, in the
1 kV range, comes from a transformer built with a small
magnetic tore triggered by an abrupt discharge of the
capacitor C2 (see Figure 2).
T1
+
C2
4.7 F/
300 V
2
GND
Figure 2. Basic Xenon Flash
The net advantage of such a concept is the very short
pulse, making easy snap shot photos to capture moving
stuffs. The drawbacks are the large physical size of the
reservoir capacitor and the recycle time needed to recharge
the capacitor between two shots (in the 5 sec range for
consumer applications). Clearly, these drawbacks make the
xenon based flash not suitable for hand held cellular phone,
with limited size and energy supply.
I
WHITE LED FLASH
V
To overcome the physical size limitation, the flash
concept is to make profit of the high efficiency, in term of
light, coming from the modern white LED.
With a 4 V forward drop voltage, such diodes do not need
extra high voltage trigger pulse, they are extremely fast to
turn ON/OFF and all the associated electronic circuit can be
housed inside a standard portable phone. Since the white
LED have electrical characteristics similar to the standard
LED (see Figure 3), one must provide a constant forward
current to control the device.
VCC
Vtrig
Figure 1. Xenon Flash Breakdown Voltage
The gas is ignited and the plasma generates a bright flash,
the typical duration being 2 ms for consumer applications.
October, 2003 − Rev. 0
X1
S1
XENON LAMP CONCEPT
 Semiconductor Components Industries, LLC, 2003
1
R1
470 k
1
Publication Order Number:
AND8135/D
AND8135/D
The circuit, built around the NCP5007, is designed to
support both the low beam current and the high flash pulse
as requested during the capture of a photo.
The DC/DC boost converter, associated to the sense
resistor R1, provides a constant current to the load to
properly bias the white LEDs. With an internal 200 mV
voltage reference (Vref), the chip minimizes the drops along
the battery supply path.
IF
100 mA
10 mA
−5 V
VF
VRR
3.5 V
−1 A
4.0 V
Low Power Beam Operating Mode
Generally speaking, this mode of operation is used to
pre−light the scene to be capture in order to minimize the
red−eye effect.
The NMOS transistor Q1 is biased OFF and R1 provides
the feedback voltage to regulate the load current. The value
of R1 is derived from the Ohm’s law:
IR
Figure 3. Typical White LED Characteristics
Consequently, a standard voltage source cannot be used
straightforward and an extra ballast is necessary to set up the
current. On the other hand, the flash must be capable to
operate over the typical battery voltage spread (2.8 V to 5.2
V) and a more suitable structure than a simple linear voltage
regulator is mandatory. To achieve such constraints, ON
Semiconductor has developed a full family of white LED
drivers, among which the NCP5007 can fulfill the flash
application demands.
V
R1 ref
Iout
With a typical 4 mA operating bias of the LED during the
illumination of the scene, the sense resistor is 51 Ω. The
current can be dynamically modulated, if necessary, by
using the EN signal pin 3 as a digital control: such a mode
of operation is depicted in the NCP5007 data sheet. The
same pin can be used to control the DC/DC by a bit from the
external CPU. Of course, a more powerful light can be
provided by setting the sense resistor accordingly.
From a practical stand point, capacitor C2 is mandatory to
avoid large spikes during the energy transfers from the
inductor L1 and the white LEDs. Moreover, such a capacitor
smoothes the current flowing into the LEDs, yielding a
better light efficiency.
TYPICAL FLASH APPLICATION
Since the battery voltage ranges from a low 2.8 V to a high
5.2 V, the simplest and economic way to handle this span is
to arrange the white LED in series as depicted Figure 4. Such
a layout avoid the leakage current during the stand by mode
operation (most of the time, the flash is not activated!).
VBat
U1
3
ENABLE
GND
2
1
EN VBat
C1
4.7 F
5
L1
22 H
Vout
FB
GND
D1
GND
4
D4
D3
D2
R10
3.3 GND
Q1
FLASH
C2
10 F/
16 V
MBR0530
NCP5007
LWT67C LWT67C
LWT67C
R1
51 GND
Figure 4. Typical Portable White LED Flash Circuit
http://onsemi.com
2
AND8135/D
High Power Flash Mode
an external resistor in series as depicted in the demo board
schematic diagram. Table 1 gives a selection of the preferred
product to handle such a function.
Once the system is ready to take the photo, the flash is
activated by forcing the high current bias into the white
LEDs. Unlike the silver film, the electronic sensor of a
digital camera cannot capture the scene is a couple of
millisecond, but a much longer delay is necessary to save all
the pixels. Typically, such a delay ranges from 100 ms to 200
ms, depending upon the type of camera and lens.
Consequently, using a reservoir capacitor to supply the large
current during 200 ms is not really feasible. In fact, to sustain
100 mA during 200 ms, with three LEDs in series, assuming
the voltage cannot drop more than 0.5 V during the pulse,
one should have a 40000 µF/16 V electrolytic capacitor, a
value not compatible with a portable equipment. On top of
that, to re−charge this capacitor in a reasonable time,
typically one second, the DC/DC converter should provide
around 500 mA when loading the capacitor from zero.
Therefore, instead of designing a chip to re−charge a
capacitor, it is far better to use the converter to supply
immediately the current called by the application.
To activate the flash, one shall turn ON the NMOS Q1,
providing a lower sense resistor in the feedback loop. The
NMOS is selected with an internal Rdson suitable for the
expected current. Since the Rdson of the NMOS varies
largely with the temperature and the spread from one lot to
another one is relatively large, one can get a more
predictable circuit by using a larger NMOS associated with
Table 1. Preferred NMOS Products
Part
Icmax
Rdson
Package
BVSS
MMBF0201NLT1
6A
35 mΩ
SO−8
30 V
MMBF0201NLT1
0.2 A
1Ω
SOT−23
20 V
MMBF2201NT1
0.2 A
1Ω
SC−70
20 V
NTA4001NT1
0.24 A
1.5 Ω
SC−75
20 V
MMFT960T1
0.3 A
1.7 Ω
SOT−223
60 V
One can use a NPN bipolar device to fulfill this function,
but the saturation voltages (Vcesat) of such devices is close
to the 200 mV define by the internal reference, and cannot
be easily implemented in this type of circuit.
DEMO BOARD SCHEMATIC DIAGRAM
The demo board, depicted in Figure 10, supports the low
beam and the high power flash, together with a digital PWM
circuit to dim the LED. On top of that, a built−in clock
provides the capability to generate multiple flash for
evaluation purpose. The mode of operation is selected by
means of switches S1 and S2, associated to potentiometers
P1 to P4 as depicted Table 2.
The third switch S3 is a push button to manually trig the
flash.
Table 2. Switches Configurations and Potentiometers Functions
S1
Select the NCP5007 mode of operation:
GND = EN pin 3 forced to High, DC operation
VCC = EN pin 3 pulsed
If S1 = VCC, then dim the light out of the LED :
P1 = Adjust the pulse width applied to EN pin 3
P2 = Adjust the PWM frequency
S2
Select the Power Flash mode of operation:
GND = Single shot triggered by S3
VCC = Repetitive mode
If S2 = VCC, then
P3 = Adjust the power flash repetitive frequency
P4 = Adjust the power flash duration
S3
Manual switch to trig the power flash
The system can be re−arranged to either dim the light
when running the low beam current, or to generate a pulsed
flash at a low pace.
The dimming function is activated when switch S1 is
High. In this mode, the clock built with gates U3, associated
with the one shot circuit U1, controls the EN pin 3, thus a
PWM modulation of the DC load current.
The pulsed flash is activated when switch S2 is High. In
this mode, the clock built with gates U3 and U4 , associated
with the second side of the one shot U1, provide a low rate
to trig the flash. The pulse width can be manually adjusted
with potentiometer P4.
The waveforms captured from the demo board (see Figure
5 to Figure 9), illustrate the currents and voltages across the
major points.
The extra functions make possible the operation of the
demo board on a stand alone basis, without any need from
external control. However, in order to provide higher
flexibility, provisions are made to connect the demo board
to a MPU: the three pins connector shall be connected to the
appropriate port to control the LED dimming and flash
functions.
In the stand alone operation, switches S1 and S3 are forced
to Low and the chip runs continuously. The low beam
current is set up by the sense resistor R1: the demo board
comes with a 51 Ω resistor, yielding 4 mA of DC current
through the white LED. At this point, one can trig the flash
by pushing switch S3. The flash is pre−set to provide 60 mA
during the 200 ms time adjusted by potentiometer P4.
http://onsemi.com
3
AND8135/D
Figure 5. Low Beam Inductor Current
Figure 6. High Power Flash Inductor Current
http://onsemi.com
4
AND8135/D
Figure 7. Input Supply Current
Figure 8. Low Beam & High Beam Output Current
http://onsemi.com
5
AND8135/D
Figure 9. Combined Low Beam & Power Flash Output Current & Output Power
http://onsemi.com
6
AND8135/D
C6
4.7 F/16 V
C2
4.7 F/6 V
GND
2
1
CLR
Q
5
12
Q
9
TRA
10
11 TRB
6
7
CTC
RCCOM
Adjust Flash
Pulse Width
U1B
M54HC123
SNJ54HC132J
(14)
10
U3C
1N4148
1N4148
NL27WZ14
4
U4B
3
13
EXTL_EN
6
U4A
11
R8
100k
GND
R14
1.5k
GND
GND
VCC
REPEAT
TRIG
GND
Figure 10. Demo Board Schematic Diagram
http://onsemi.com
GND J2
EXTERNAL
3
2
1
S3
GND
7
1
R6
10k
P3
100kA
U3D
Adjust Flash Duty Cycle
10 F/10 V
NL27WZ14
C4
EXTL_FLASH
1 nF
SNJ54HC132
C7
R3
10k
D9
PWM
VCC
D8
MBR0530
GND GND
S1
NORMAL/PWM
R13
1.5k
VCC
D2
D7
NORMAL: Constant DC Current
PWM: dim LED intensity
4.7 F/16 V
GND VCC
GND
SNJ54HC132
5
GND
SNJ54HC132
6
U3B
4
3
U3A
1
R7
100k
2
Adjust PWM
R2
10k
100 nF
C9
J1
Vbat
D6
VCC
P1
500kA
C10
9
R4
10k
P2
100kA
C8
1F/10V GND
8
VFB
GND
100 nF
C11
6
R9
51R
100 nF
VCC
R5
10k
12
2
NL27WZ32
2
4
TP1
C3
R1
10k
P4
500kA
GND
GND
Q
CLR
U1A
M54HC123
1
C1
VCC
Q1
MMBF0201NLT1
VCC
100 nF
5
3
GND
4
Vout
1
FB
R10
3.3R
1
TRA
2 TRB
3
14
15
CTC
RCCOM
Q
13
U5A
7
3
ENVbat
U2
NCP5007
5
D3
LWT67C
U5B
NL27WZ32
D4
LWT67C
VCC
GND
GND
GND
D5
LWT67C
MBR0530
D1
L1
22 H
R11
10k
R12
10k
C5
4.7 F/16 V
S2
SINGLE/
REPEAT
AND8135/D
Figure 11. PCB Layout
http://onsemi.com
8
AND8135/D
Table 3. High Beam Demo Board Part List
Used
Part
Designator
Footprint
Description
2
1.5 kΩ
R13, R14
0805
Resistor
1
3.3 Ω
R10
0805
Resistor
1
51 Ω
R9
0805
Resistor
8
10 kΩ
R1, R2, R3, R4, R5, R6,
R11, R12
0805
Resistor
2
100 kΩ
R7, R8
0805
Resistor
2
1N4148
D6, D7
DIODE0.4
Diode
2
100 kΩ
P2, P3
VR4
Potentiometer, Linear
1
500 kΩ
P1
VR4
Potentiometer, Linear
1
500 kΩ
P4
VR4
Potentiometer, Linear
4
100 nF
C1, C3, C9, C10
0805
Ceramic Capacitor, MURATA
1
10 µF/10 V
C4
1210
Ceramic Capacitor, MURATA
1
1 µF/10 V
C8
0805
Ceramic Capacitor, MURATA
1
1 nF
C7
0805
Ceramic Capacitor, MURATA
4
4.7 µF/6 V
C2, C5, C6, C11
1210
Ceramic Capacitor, MURATA
2
LED
D8, D9
LED_2
LED
3
LED
D3, D4, D5
LED_2
LED: OSRAM LWT67SQ2−4
1
22 µH
L1
1210
Inductor: CoilCraft 1008
1
M54HC123
U1
SO−16
Dual Retriggerable one shot
2
MBR0530
D1, D2
1210
Schottky Diode
1
SNJ54HC132
U3
SO−14
Quadruple Positive−NAND Gate
with Schmitt−Trigger Input
1
MMBF0201NLT1
Q1
SOT−23
MOSFET
1
NCP5007
U2
TSSOP5
White LED driver
1
NL27WZ14
U4
SOT_23B
Dual schmitt trigger inverter
1
NL27WZ32
U5
US8
Dual OR gate
1
EXTERNAL
J2
SIP3
Connector
1
GND
Z1
GND_TEST
Connector
1
NORMAL/PWM
S1
SIP3
Manual Switch
1
SINGLE/REPEAT
S2
SIP3
Manual Switch
1
TRIG
S3
PUSH_BUT_B
Push Button
1
VFB
TP1
TEST_POINT
Connector
1
Vbat
J1
PLUG_4MM_DUAL
Connector
http://onsemi.com
9
AND8135/D
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should
Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,
and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death
associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal
Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
Literature Distribution Center for ON Semiconductor
P.O. Box 5163, Denver, Colorado 80217 USA
Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada
Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada
Email: [email protected]
N. American Technical Support: 800−282−9855 Toll Free
USA/Canada
ON Semiconductor Website: http://onsemi.com
Order Literature: http://www.onsemi.com/litorder
Japan: ON Semiconductor, Japan Customer Focus Center
2−9−1 Kamimeguro, Meguro−ku, Tokyo, Japan 153−0051
Phone: 81−3−5773−3850
http://onsemi.com
10
For additional information, please contact your
local Sales Representative.
AND8135/D